NATURAL RESOURCES
CORNELL COOPERATIVE EXTENSION

Nitrogen: The Essential Element

by

Nancy M. Trautmann and Keith S. Porter
Center for Environmental Research
and
Robert J. Wagenet
Dept. of Agronomy
Cornell University

Use of Nitrogen as a Fertilizer

In 1898, the president of the British Association for the Advancement of
Science, Sir William Crookes, startled a distinguished scientific audience when
he declared during his presidential address that "England and all civilized
nations stand in deadly peril of not having enough to eat." The deadly peril
that Sir William foresaw was the inability of farmers to satisfy the increasing
demand for food given current supplies of nitrogen.

At the time of Sir William's address, the main sources of nitrogen fertilizers
were sodium nitrate and ammonium sulphate. Sodium nitrate was obtained from
immense deposits of nitrate-bearing rocks, called caliche, that had been
discovered in Chile at the beginning of the nineteenth century. Ammonium sulfate
was obtained from coal gas. Other sources of nitrogen included sewage, guano
(bird droppings), and manure, but these were of declining importance.

Sir William suggested that to meet the world's increasing nitrogen needs,
chemists must develop methods for artificially fixing atmospheric nitrogen. A
successful response to this challenge was made by the German scientist Fritz
Haber. Haber created a method for synthesizing ammonia from nitrogen and
hydrogen, which was later developed into an industrial process by the industrial
chemist Carl Bosch and became known as the Haber-Bosch method.

At the outset of the First World War, the estimated world annual production of
nitrogen fertilizers was about 0.6 million tons, compared to world production of
2.5 and 1.0 million tons of phosphorus and potassium fertilizers, respectively.
Today, primarily because of the Haber-Bosch process, this trend has been
reversed. Currently, the United States alone annually produces about 12 million
tons of nitrogen fertilizers, compared with 10 million tons of phosphorus and
only 2 million tons of potassium fertilizers.

With today's high inputs of nitrogen fertilizers, American farmers routinely
achieve levels of crop productivity that would have seemed improbable to Sir
William Crookes and his fellow scientists. In the past fifty years, for example,
the average annual yields of corn in the United States have increased by a
factor of four or five. This advance in agricultural productivity, and the
fertilizer use upon which it depends, is indispensable if the world's population
is to be sustained. Unfortunately, these gains have incurred environmental
costs; some nitrogen applied to crops escapes to ground and surface waters,
sometimes with damaging consequences.

Nitrogen in Plants, Soil, and Groundwater

Good crop yields depend on an adequate supply of nitrogen. Most non legume crops
require added nitrogen to achieve the yields required today. Lacking sufficient
nitrogen, plants usually become yellow and stunted, with smaller than average
flowers and fruits. For example, grain crops grown with inadequate nitrogen
produce a poor yield with low protein content. Without nitrogen fertilizers, an
estimated one-third of our current agricultural production would be lost.

Under most conditions, however, farmers supply more than twice the nitrogen
required by a crop to achieve the best yields. Unfortunately, much of the
applied nitrogen is mobile in soil and may be carried to groundwater, possibly
contaminating drinking water supplies. (See Cornell Cooperative Extension Fact
Sheet 400.02, Nitrate Health Effects in Drinking Water, for a discussion of this
issue.) Understanding the chemistry of nitrogen in soils can help farmers supply
sufficient nitrogen for crop needs without losing excessive amounts to
underlying groundwater.

Forms of Soil Nitrogen. Nitrogen occurs naturally in many forms. In the soil, it
exists in two major classes of compounds:

Organic nitrogen, such as proteins, amino acids, and urea, including nitrogen
found within living organisms and decaying plant and animal tissues.

Within these two forms, there are many different nitrogen compounds. Some are
soluble and others are relatively insoluble; some are mobile in soil and others
are immobile; and some are available for plant uptake and others are not.
Nitrogen in soil is continually being transformed among these various forms
through a complex network of physical, chemical, and biological reactions
collectively called the nitrogen cycle.

The Nitrogen Cycle. The nitrogen cycle in soil includes the following processes,
in which microbes play a crucial role (fig 1. See fact sheet):

Fixation. Ninety percent of the earth's nitrogen is in the atmosphere in
the form of dinitrogen gas (N2). Gaseous nitrogen is chemically stable and
unusable by most biological organisms. Some species of bacteria absorb
atmospheric dinitrogen gas and convert it into ammonium, which plants can use.
This process, called nitrogen fixation, is the principal natural means by which
atmospheric nitrogen is added to the soil.

Mineralization. As plant and other organic residues decompose, nitrogen
is converted to ammonium by soil microorganisms through a process known as
mineralization. Plant roots absorb some of the ammonium, and some is chemically
converted to gaseous ammonia and lost to the atmosphere.

Nitrification. Bacteria transform the ammonium in the soil to nitrite and
then to nitrate in a sequence of steps called nitrification.

Plant uptake. Nitrate is a negatively charged anion and therefore usually
remains in the soil water rather than being adsorbed to soil particles. Plants
readily absorb nitrate through their roots and use it to produce protein.

Leaching. The nitrate not captured by plant roots is free to move with
soil water. This can result in significant leaching, or movement of the nitrate
to deeper soil depths.

Denitrification. Where there is a deficit of oxygen in the soil, called
an anaerobic condition, some bacteria meet their energy needs by reducing
nitrate to dinitrogen gas or to nitrogen oxide (N2O). This biological process is
called denitrification. It results in a loss of nitrogen from the soil and the
return of nitrogen to the atmosphere.

Fate of Nitrogen in the Field

In the soil of any farm field, nitrogen is in a continuous state of flux. Losses
occur when crops are removed for livestock feed or human food, which often is
consumed far from the land on which it was produced. Surface runoff and
consequent soil erosion can also cause significant losses of soil nitrogen.
Other losses occur through volatilization of ammonia and leaching or
denitrification of nitrate.

Three types of inputs can compensate for nitrogen losses in farm fields: (1)
fertilization, (2) nitrogen fixation by legumes, and (3) supplementation with
manure or other organic matter high in nitrogen. Farm management of soil
nitrogen depends on an understanding of these inputs and outputs so that crop
needs can be adequately met without excessive nitrogen losses to the
environment.

Gaseous Losses of Nitrogen. In cultivated fields, nitrogen is converted into gas
and released into the atmosphere in two ways. First, when urea and ammonium
forms of fertilizers (such as anhydrous ammonia, ammonium nitrate, ammonium
sulfate, and ammoniated phosphates) are deposited on moist surfaces, they may
undergo a series of chemical conversions to ammonia. The ammonia gas then
escapes to the atmosphere rather than becoming a plant nutrient. This loss,
termed volatilization, is reduced if the fertilizer is washed into the soil by
rain or irrigation or if the fertilizer is drilled into the soil to a depth of
an inch or more.

The second route by which nitrogen is lost to the atmosphere is through
denitrification. If pockets in the soil become saturated with water so that
oxygen is excluded, denitrifying bacteria can reduce the nitrate to dinitrogen
or nitrogen oxide gas. Poorly drained and heavy soils are particularly prone to
denitrification, and a substantial amount of applied nitrogen may be lost to the
atmosphere.

In some nonagricultural cases, denitrification is beneficial. In septic tanks
and leaching fields, for example, denitrification releases nitrogen to the air
as a gas, reducing the amount of nitrate available for potential contamination
of ground and surface waters.

Conversion of Nitrogen to a Plant-Available Form. Only inorganic nitrogen can be
absorbed by plants. The greater part of nitrogen in the field, however, is
usually in organic form such as proteins and amino acids. Under normal
conditions in the northern hemisphere, only about 2 or 3 percent of the organic
nitrogen in soil is converted to inorganic nitrogen each year. The natural decay
of organic matter provides a slow but continuous supply of nitrogen, which tends
to be taken up by plants rather than lost to the atmosphere or to water.

Legumes can supplement soil nitrogen supplies by fixing nitrogen from the
atmosphere. This is accomplished by nitrogen-fixing bacteria living in nodules
on the plant roots.

Leaching of Nitrate. Nitrate does not adsorb strongly to soil particles. If not
taken up by plants, nitrate will be either denitrified or carried below the root
zone, perhaps to groundwater. Factors that determine whether nitrate will reach
groundwater include:

the amount of nitrate in the soil, the quantity and timing of rainfall or
irrigation, the soil's capacity to hold water, the presence and density of
plants, the rates of infiltration and percolation of water through the soil, the
rate of evapotranspiration relative to precipitation and irrigation, and the
soil temperature.

In the eastern United States, the opportunity for nitrate leaching is greatest
in early spring and in the fall. Rainfall tends to be frequent and heavy during
these seasons, and the low rates of plant growth and evapotranspiration permit
more of the added water to percolate downward to groundwater. Plant uptake of
dissolved nutrients also is low during these periods, so leaching losses tend to
be high. Soil type is a major factor influencing the degree to which nitrate is
lost to groundwater. Sandy or other very well drained soils are most vulnerable
to leaching losses.

Farmers can do little to change the character of the soils in their fields.
Likewise, they have no control over the vagaries of the weather. What, then, can
farmers do to conserve a valuable crop nutrient while minimizing nitrate
contamination of groundwater?

Fertilize Crops, Not Groundwater

To use fertilizer nitrogen correctly, take the following steps for non legume
crops:

Establish a realistic goal for crop yield,and from this goal estimate the amount
of nitrogen that the crop must accumulate.

Estimate the amount of nitrogen that will be supplied by the mineralization of
soil organic nitrogen and crop residues.

Use any available manure nitrogen to supplement the soil and crop residue
supplies.

If necessary, supplement these nitrogen sources with enough fertilizer to meet
the yield goal for the particular crop.

Apply any needed fertilizer just before the period of most rapid crop uptake to
minimize leaching and denitrification.

These steps are fully described in the Cornell Field Crops and Soils Handbook,
the 1990 Cornell Recommends for Field Crops, and two fact sheets by Klausner and
Bouldin (see "For Further Reading" for reference information).

One method of increasing the efficiency of fertilizer use and decreasing the
amount lost to groundwater is to delay a portion of the nitrogen application
until the crop is growing rather than applying it all at the time of planting
(fig. 2. See fact sheet). Field experiments have shown that splitting fertilizer
applications can increase the efficiency of nitrogen use, maintain crop yields,
and decrease fertilizer costs.

Another technique for meeting crop needs while decreasing leaching losses is to
supply nitrogen in an organic form such as manure or legume residues. Because
organic nitrogen is gradually converted into inorganic nitrogen, only small
amounts are in a soluble form susceptible to leaching at any one time. It is
estimated that the amount of atmospheric nitrogen taken up by legumes roughly
equals the amount removed by harvesting. If harvested hay is removed from the
farm, then soil nitrogen should remain approximately constant. If the hay is fed
to animals on the farm, about one-half of the nitrogen can be returned to the
fields if the manure is handled carefully.

To avoid excessive nitrogen losses through volatilization, runoff, and leaching,
follow these procedures in handling manures:

Collect manure as soon as possible after it is deposited, and conserve the
liquid portion.

Store manure under conditions that prevent drying or drainage

Apply manure to the field close to the time of planting, so that the available
nitrogen will be taken up by plants rather than leached, lost to the atmosphere,
or converted to organic forms.

Plow manure under as soon as possible after spreading to minimize ammonia
volatilization.

(See the two fact sheets by Klausner and Bouldin for more information on manure
management.)

Conclusions

Nitrogen is likely to be in short supply for crop production unless supplemented
by legume crop residues or by the application of fertilizers, manures, or other
high-nitrogen materials. Nitrogen fixing microbes replenish soil nitrogen by
converting the relatively inert nitrogen of the atmosphere into a form that can
be used by living organisms. Since biological fixation of nitrogen is not
usually sufficient to meet the needs of intensive crop production, however,
additional sources may be needed.

Fertilization schemes should be designed to meet crop nitrogen needs without
losing excessive amounts of nitrogen to groundwater. This is accomplished by
estimating nitrogen needs and meeting these needs as much as possible with
manure or other organic sources.

The greatest potential for nitrate leaching occurs if fertilizer is applied at a
time when no crops are growing, such as during spring planting or in the fall
after harvest. Leaching losses can be reduced by applying nitrogen in increments
during the periods of rapid plant growth. Using manure or slow- release
fertilizers also limits leaching.losses because plant-available (and leachable)
nitrogen is supplied gradually rather than all at once. Even with these sources,
however, leaching can occur if the nitrogen supplied exceeds the ability of the
crop to use it.

Growing legumes and using organic soil amendments enrich the soil with organic
nitrogen, which does not leach and provides a continuous supply of plant-
available nitrogen as it slowly decomposes. Adding organic matter to soil
provides additional benefits by enhancing the soil's ability to retain water and
dissolved nutrients in the root zone where they are available to plants.
Protecting organic topsoils from erosion, providing organic soil amendments, and
managing water and fertilizer applications for maximum plant uptake will lead to
efficient fertilizer use and protection of groundwater quality.

Modern Agriculture. Its Effects on the Environment (400 01) Nitrate: Health
Effects in Drinking Water (400.02) Pesticides: Health Effects in Drinking water
(400.03) Groundwater: What It Is and How to Protect It (400.04) Water and the
Soil (400.05) Pesticides and Groundwater: A Guide for the Pesticide User
(400.07)

Acknowledgments: Illustrations were drawn by Christine Cleveland and Donna
Curtin, and Mary Jane Porter served as production assistant. Funding was
provided by the New York Farmers' Fund. Many individuals reviewed the initial
drafts, including Cornell University faculty members, northeastern Cornell
Cooperative Extension agents, and employees of the U.S. Department of
Agriculture and the U.S. Geological Survey. Special thanks are due to Professor
David Bouldin for his technical input and review.